European Journal of Clinical Nutrition (2002) 56, Suppl 3, S46–S49 ß 2002 Nature Publishing Group All rights reserved 0954–3007/02 $25.00 www.nature.com/ejcn ORIGINAL COMMUNICATION The thymus gland is a target in malnutrition W Savino1,2* 1Laboratory on Thymus Research, Department of Immunology, Oswaldo Cruz Institute, Oswaldo Cruz Foundation, Rio de Janeiro, Brazil; and 2CNRS UMR 8603, Universite´ Paris V, Hoˆpital Necker, Paris, France Malnutrition, secondary to deficiency in uptake of proteins, metal elements or vitamins, consistently results in changes in the thymus gland. The organ undergoes a severe atrophy due to apoptosis-induced thymocyte depletion, particularly affecting the immature CD4þ CD8þ cells, as well as a decrease in cell proliferation. Such a feature is apparently linked to a hormonal imbalance, involving decrease of leptin and consequent raise of glucocorticoid hormone levels in the serum. Interestingly, this picture can be reversed after appropriate diet rehabilitation. The thymic microenvironment is also affected in malnutrition: morphological changes in thymic epithelial cells were found, together with a decrease of thymic hormone production by these cells. Additionally, intrathymic contents of extracellular proteins, such as fibronectin, laminin and collagens, are increased in the thymuses from malnourished children. Conjointly, the bulk of data discussed herein clearly points to the notion that the thymus gland is a target in malnutrition. Nevertheless, further relevant information regarding the physiology of the thymus, including the cytokine=chemokine secretion as well as the positive and negative selection events driven by TCR=MHC-peptide interactions in malnutrition, remains to be defined. These are questions that need to be answered in order to have a better understanding of the immunodeficiency seen in malnourished individuals. European Journal of Clinical Nutrition (2002) 56, Suppl 3, S46 – S49. doi:10.1038=sj.ejcn.1601485 Keywords: thymus; malnutrition; thymocytes; thymic epithelium; thymulin; extracellular matrix; glucocorticoid hormones Introduction ultimately leading to migration of positively selected thy- It has been a long time since scientists noticed that, in the mocytes to the T-cell-dependent areas of peripheral lym- context of the malnutrition-related immunodeficiency, the phoid organs. Such a process involves sequential thymus undergoes a variety of alterations, with a severe expression of various proteins and rearrangements of the T- atrophy of the organ representing one major change cell receptor (TCR) genes. The most immature thymocytes (reviewed in Chandra, 1992). This is so consistent that the express neither the TCR complex nor the CD4 or CD8 thymus is accepted as a barometer of malnutrition. accessory molecules; they are called double-negative thymo- As detailed below, distinct cellular compartments of the cytes, and represent 5% of total thymocytes. organ are affected in malnourished individuals, with Maturation progresses with the acquisition of CD4 and changes in various aspects of thymic cell behaviour such as CD8 markers, generating the CD4þ CD8þ double-positive proliferation, death and secretion. Nevertheless, before com- cells, which comprise 80% of the whole population. In piling and discussing these data, it is worthwhile to provide a this stage, TCR genes are rearranged, and productive general background of the thymic microenvironment and its rearrangements yield the membrane expression of the role upon intrathymic T cell differentiation. TCR (complexed with the CD3) in low densities (TCRlow). Thymocytes that do not undergo a productive TCR gene rearrangement die by apoptosis, whereas those The thymic microenvironment and its role in T-cell expressing productive TCR will interact with peptides differentiation presented by molecules of the major histocompatibility The thymus is a primary lymphoid organ, in which bone complex (MHC), expressed on microenvironmental cells. marrow-derived T-cell precursors undergo differentiation, This interaction will determine the positive and negative selection events, crucial for normal thymocyte differentia- tion. Positively selected thymocytes progress to the mature TCRhigh CD4þCD87 or TCRhigh CD47CD8þ *Correspondence: W Savino, Hoˆpital Necker, CNRS UMR 8603, Universite´ Paris V, 161 rue de Se`vres, 75743 Paris Cedex 15, France. single positive stage, comprising 15% of thymocytes E-mail: [email protected] that ultimately leave the organ to form the large majority Thymus gland and malnutrition W Savino S47 of the peripheral T cell repertoire (reviewed in Savino & cortical thymocyte depletion is a consistent finding in Dardenne, 2000). necropsies of malnourished subjects (Chandra, 1992; Lyra Thymocyte differentiation occurs as cells migrate within et al, 1993). Indeed, by means of echography, atrophy of the the thymic lobules: TCR7CD47 CD87 and TCRþCD4þCD8þ organ was also observed in vivo in malnourished children are cortically located, whereas mature TCRþ CD4þCD87 and (Parent et al, 1994). Nevertheless, such alterations seem to be TCRþCD47CD8þ cells are found in the medulla. Along this reversible if appropriate diet is provided. This concept journey, thymocytes interact with various components of emerges from an interesting longitudinal study carried out the thymic microenvironment, a tridimensional network with Bolivian severely malnourished children that were formed of thymic epithelial cells (TEC), macrophages, den- treated for nutrition rehabilitation (Chevalier et al, 1996). dritic cells, fibroblasts and extracellular matrix components. Thymus size was assessed weekly by mediastinal ultrasound In addition to the key interaction, involving the TCR=pep- scanning. Compared to controls, the malnourished group peptide-MHC, in the context of CD8 or CD4 molecules, the had severe involution of the thymus, a significantly higher thymic microenvironment influences thymocyte maturation proportion of circulating immature T lymphocytes and a via adhesion molecules and extracellular matrix (ECM); lower proportion of mature T lymphocytes. The 2 month interactions that are relevant for thymocyte migration. longitudinal study showed that normal anthropometric Moreover, microenvironmental cells modulate thymocyte values were recovered after 1 month of diet rehabilitation, differentiation by soluble polypeptides, comprising: (a) cyto- whereas recovery of the thymic area required 2 months. This kines, such as interleukin-1 (IL-1), IL-3, IL-6, IL-7, IL-8 and may explain the frequent relapses among some malnour- stem cell factor; (b) chemokines; and (c): thymic hormones, ished children, discharged after 1 month on the basis of including thymulin, thymopoietin and thymosin-a1 (Savino apparent nutritional health. Thus, the evaluation of the & Dardenne, 2000). thymus can be placed as an interesting tool for diagnostic= prognostic of immune recovery in humans treated for malnutrition. Phenotypic and functional changes of thymocytes in malnutrition As stated above, one of the most conspicuous changes in Is there a hormonal control of the malnutrition — malnutrition is thymic atrophy. This phenomenon is largely induced thymocyte depletion? (but not exclusively) due to changes in the lymphoid com- It is now well established that the physiology of the thymus partment. In fact, thymocyte depletion appears as an out- (including both lymphoid and microenvironmental com- come of both acute and chronic experimental protein partments) is under neuroendocrine control (Savino & Dard- malnutrition. The main phenotypic feature of this depletion enne, 2000). In this context, it is conceivable that distinct is the loss of immature CD4þCD8þ cells, a finding consis- pathological states in the organism can affect such a hormo- tently seen in malnutrition secondary to protein, metal nal influence upon the thymus. It has been shown that element — including zinc, magnesium and iron — or vitamin glucocorticoid-circulating levels are increased in protein- deficiency diets (Chandra, 1992; Kuvibidila et al, 1990; Dhur malnourished mice, as compared to age-matched controls. et al, 1991; Malpuech-Brugere et al, 1999). As recently Additionally, implanted corticosterone-containing pellets, demonstrated in rats exposed to deficiencies of Mg or Zn, able to generate glucocorticoid serum levels equivalent to the consequent thymocyte depletion actually reflects a mas- those found in malnourished mice, were sufficient to yield sive apoptosis of these cells in the organ (Malpuech-Brugere thymocyte depletion (Barone et al, 1993). Accordingly, it was et al, 1999; Nodera et al, 2001). demonstrated that the atrophy of the organ was prevented in In addition to the increase in thymocyte death seen in animals that were adrenalectomized prior to being exposed thymuses from malnourished individuals, thymocyte prolif- to a zinc-deficient diet (Fraker et al, 1995). eration seems to be affected, since the numbers of thymic It is noteworthy that in mice submitted to acute starva- cells labelled with PCNA (proliferating cell nuclear antigen) tion, in addition to the severe thymocyte depletion and rise marker are diminished in malnourished rats (Mitsumori et al, in serum corticosterone, a significant diminution in the 1996). This finding is further supported by the data showing levels of the adipocyte-derived hormone leptin has been that thymocytes from animals subjected to distinct protocols detected. Furthermore, injection of recombinant leptin of diet restriction exhibited a low mitogen-induced prolif- could